Mark Zuckerberg's inflated experiment in monetising data harvesting, Facebook, has taken another step down the rabbit hole. Now it will open up its database to marketers who can use personal information that they have accrued, such as phone numbers and email addresses, to find potential customers.

Facebook's clause is that the company must already have some of that personal data on their records, or, as AllThingsDigital puts it, Zuckerberg is enabling marketing departments to hunt down customers, present or prior, on the social network. Advertisers will be able to use the data to stalk people across the network as soon as next week.

This poses some niggling questions. Anyone with a telephone will understand that, even with opt-out services to hide your number from the public record, all it takes is for one unscrupulous marketer to get their hands on it and it becomes public domain for the cold callers and scammers.

Hidden in terms and conditions of countless email sign-ups or member sites are clauses that say they may pass over information to relevant third parties, with your consent, upon joining.

Sometimes this is as easy to do as accidentally leaving a box un-ticked on a sign-up form, others pass and sell data without an obvious record. To opt out of such services is the receiver's responsibility, not the sender's.

Of course, Facebook has had its fair share of criticism flung its way, and it claims that advertisers must have the permission of their customers for using that data before they can go ahead and track them. The question lies in when consent is muddied or hidden.